Dark Molecular Gas in Simulations of z~0 Disc Galaxies

TL;DR

This study uses simulations to explore the amount and properties of CO-dark molecular gas in nearby disc galaxies, predicting how alternative tracers like [CI] and [CII] can effectively trace this hidden gas component.

Contribution

It combines cosmological simulations with ISM modeling to quantify CO-dark gas and assess the effectiveness of [CI] and [CII] as tracers, providing new insights into molecular gas detection.

Findings

Over 50% of H2 gas is CO-dark and diffuse.

[CII] predominantly traces CO-dark molecular gas.

[CI] is a reliable tracer with minimal contamination.

Abstract

The $\rm H_2$ mass of molecular clouds has traditionally been traced by the CO(J=1-0) rotational transition line. This said, CO is relatively easily photodissociated, and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark". We investigate the amount and physical properties of CO-dark gas in two $z \sim 0$ disc galaxies, and develop predictions for the expected intensities of promising alternative tracers ([CI 609 $\mu$m and [CII] 158 $\mu$m emission). We do this by combining cosmological zoom simulations of disc galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H~\textsc{i} and $\rm H_2$ abundances and CO(J=1-0), [CI] 609 $\mu$m and [CII] 158 $\mu$m emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field and cosmic ray ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e. $\rm H_2$ gas with $W_{\rm CO} < 0.1 $ K-km/s, we find that a significant amount ($> 50\%$) of the total $\rm H_2$ mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [CII], though [CI] also serves as a bright tracer of the dark gas in many instances. At the same time, [CII] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [CI] suffers the least contamination from diffuse atomic gas, and is relatively insensitive to secondary parameters.